It is known that the appearance of an object is composed of a plurality of components such as a specular reflection component, which is the incident light reflected by the surface of the object being observed as a “gloss”, and a diffuse reflection component observed as light repeatedly scattered inside the object.
In recent years, methods have been widely employed in which an image is separated into such components for compressing the information for digital archives or for obtaining the shape/surface material of the object (e.g., Non-Patent Document 1, Non-Patent Document 2 and Non-Patent Document 3). This is because a process with a higher precision can be achieved by performing a process for each of the separated components.
As the method for separating an image into different components as described above, a method for separating it into a specular reflection component and a diffuse reflection component has been widely used. Particularly, a method utilizing polarization information has been widely researched because specular reflection and diffuse reflection can be separated with a simple device. In this method, a linearly-polarized light source is emitted onto the object, and the specular reflection component and the diffuse reflection component are separated from each other by rotating a linear polarization filter provided between the camera and the object about the optical axis.
It is known that where Φ is the rotation angle (polarization main axis angle) of the linear polarization filter, the luminance value observed in each pixel changes along a sinusoidal function with respect to the rotation angle Φ. The image separation can be done by determining the amplitude component of the sinusoidal function as the “specular reflection component” and the bias component as the “diffuse reflection component”. That is, for each of a plurality of pixels of an image, the pixel can be assigned as a “specular reflection area” or a “diffuse reflection area” depending on whether the specular reflection component or the diffuse reflection component is dominant. In other words, pixels where the specular reflection component is dominant form a “specular reflection area”, and pixels where the diffuse reflection component is dominant form a “specular reflection area”.
A method for performing an area division for the surface of an object as described above can be realized based on the difference in polarization characteristics between specular reflection and diffuse reflection.
Since the specular reflection component occurs from surface reflection, the polarization characteristics of the incident light are maintained. Therefore, it is observed as the polarized component of the brightness observed by the camera.
Since the diffuse reflection occurs through repeated scattering, the polarization characteristics of the incident light have been lost. Therefore, it is observed as the non-polarized component of the brightness observed by the camera.
These polarization characteristics are based on the following two conditions.
(Condition 1) Where linearly-polarized light is emitted, a specular reflection component is observed as a polarized component.
(Condition 2) Where linearly-polarized light is emitted, a diffuse reflection component is observed as a non-polarized component.
Referring to FIGS. 74(a)-(c), a conventional area division will be described.
FIGS. 74(b) and (c) show the specular reflection area and the diffuse reflection area, respectively, obtained by dividing the image of FIG. 74(a) by a conventional area division. That is, areas denoted by white pixels in FIG. 74(b) are those divided as “specular reflection areas”, whereas areas denoted by white pixels in FIG. 74(c) are those divided as “diffuse reflection areas”.
As seen from these images, the peripheral portion of the sphere near the occluding edge (the area A in FIG. 74(b)) is divided as a “specular reflection area”. However, specular reflection occurs in or near a regular reflection area. A specular reflection component being estimated in such an occluding edge area indicates that the separation precision is not sufficient.
Such a problem occurs as (Condition 2) is not satisfied because a portion of the diffuse reflection component is polarized. The cause of such a problem will be discussed below with reference to FIGS. 72 and 73.
FIGS. 72 and 73 are graphs showing the degree of polarization of the specular reflection component and the degree of polarization of the diffuse reflection component, respectively, for objects whose refractive indices are n=1.1, 1.3, 1.5 and 2.0 (see, for example, L. B. Wolff and T. E. Boult, “Constraining object features using a polarization reflectance model”, IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol. 13, No. 7, pp. 635-657, 1991). The horizontal axis of the graph of FIG. 72 represents the angle of incidence, and the vertical axis represents the degree of polarization, whereas the horizontal axis of the graph of FIG. 73 represents the emittance angle, and the vertical axis represents the degree of polarization. The following can be known from the figures.
The degree of polarization of the diffuse reflection component is sufficiently small except for areas where the emittance angle is sufficiently large.
The degree of polarization of the diffuse reflection component is sufficiently larger than the degree of polarization of the specular reflection component in areas where the emittance angle is sufficiently large.
That is, the following can be known.
(Condition 2) holds except for areas where the emittance angle is sufficiently large.
(Condition 2) does not hold in areas where the emittance angle is sufficiently large.
This is a major reason why the conventional area division shown in FIG. 74 failed to accurately separate the specular reflection component and the diffuse reflection component from each other.